Implication determination is to determine whether a given sentence to be determined T implies a given hypothesis H (a task). The sentence to be determined T implies the hypothesis H means that a human who reads the sentence to be determined T can infer the hypothesis H as a fact. Assume that the hypothesis H of “Prime minister K has visited United States of America” and the sentence to be determined T of “Prime minister K has visited White House” are given as objects with which the implication determination is performed. In this case, a human who reads the sentence to be determined T can infer the hypothesis H as a fact, and therefore, it is determined that the sentence to be determined T implies the hypothesis H.
A typical method of the implication determination is disclosed in Non Patent Literature 1. The method disclosed in Non Patent Literature 1 calculates a ratio of a word or a set of dependency included in the hypothesis H in common in the sentence T (hereinafter, described as a common ratio). When the calculated common ratio is equal to or more than a threshold value of the degree of similarity (a reference value) that serves as a reference, it is determined that the sentence T implies the hypothesis H. Also, Non Patent Literature 1 discloses, as a system of the implication determination, a system in which a word used when the common ratio is calculated is expanded to a range of a thesaurus and a system in which a negative form of a sentence or modality is determined.
Further, Patent Literature 1 discloses a sentence retrieval device that retrieves related sentences similar in topic. The sentence retrieval device disclosed in Patent Literature 1 determines whether sentences are related based on the similarity of topic included in each sentence. Also, the sentence retrieval device disclosed in Patent Literature 1 determines whether the sentences are initial related documents or following related documents based on a date on which each related document is announced or reported.
Note that Patent Literature 2 discloses a communication assistance device for a smooth conversation. The device disclosed in Patent Literature 2 compares an utterance to be determined with an earlier utterance, and determines an element included in the earlier utterance but not included in the utterance to be determined as new information. To be more specific, the device disclosed in Patent Literature 2 compares a linguistic structure of a sentence in the first language uttered by a first language speaker and a linguistic structure of a sentence in the second language, and extracts new information that represents a content newly uttered in the sentence in the second language. The extracted new information is preferentially output to the other party of conversation.